Touch-sensitive devices allow a user to conveniently interface with electronic systems and displays by reducing or eliminating the need for mechanical buttons, keypads, keyboards, and pointing devices. For example, a user can carry out a complicated sequence of instructions by simply touching an on-display touch screen at a location identified by an icon.
There are several types of technologies for implementing a touch-sensitive device including, for example, resistive, infrared, capacitive, surface acoustic wave, electromagnetic, near field imaging, etc. Capacitive touch-sensing devices have been found to work well in a number of applications. In many touch-sensitive devices, the input is sensed when a conductive object in the sensor is capacitively coupled to a conductive touch implement such as a user's finger. Generally, whenever two electrically conductive members come into proximity with one another without actually touching, a capacitance is formed therebetween. In the case of a capacitive touch-sensitive device, as an object such as a finger approaches the touch sensing surface, a tiny capacitance forms between the object and the sensing points in close proximity to the object. By detecting changes in capacitance at each of the sensing points and noting the position of the sensing points, the sensing circuit can recognize multiple objects and determine the characteristics of the object as it moves across the touch surface.
There are two known techniques used to capacitively measure touch. The first is to measure capacitance-to-ground, whereby a signal is applied to an electrode. A touch in proximity to the electrode causes signal current to flow from the electrode, through an object such as a finger, to electrical ground.
The second technique used to capacitively measure touch is through mutual capacitance. Mutual capacitance touch screens apply a signal to a driven electrode, which is capacitively coupled to a receiver electrode by an electric field. Signal coupling between the two electrodes is reduced by an object in proximity, which reduces the capacitive coupling.
Within the context of the second technique, various additional techniques have been used to measure the mutual capacitance between electrodes. Each of these techniques has its own capabilities, limitations, and other characteristics, and associated advantages and disadvantages from standpoints such as performance, speed, complexity, cost, and so forth. Moreover, the question of whether a characteristic of a given technique is deemed to be an advantage or disadvantage may depend on the goals of the system designer. For example, the designer of a relatively small touch screen system with low resolution and requiring only one touch detection at a time may consider a characteristic of a given sensing technique to be advantageous, while a designer of a larger touch screen system requiring high resolution and multiple simultaneous touch capability may consider the same characteristic to be a disadvantage.
We have developed additional techniques for the measurement of mutual capacitance in touch screens that system designers may wish to use as they develop new and improved touch panel systems.